The present invention relates to improvements in spray guns, and in particular to a spray gun for spraying two liquid reactants.
One type of spray gun for plural component materials is disclosed in U.S. Pat. No. 3,366,337 to Brooks et al. That spray gun is of the internal mix airless type and means are provided for intimately mixing two liquid reactants immediately prior to spraying the same. This is accomplished by impinging streams of the reactants against each other under pressure from opposed directions into the rearward end of a relatively large mixing chamber within the gun, whereby the reactants are mixed within the chamber and then discharged through an airless outlet orifice at a forward end of the chamber.
At the end of a spraying operation with the spray gun of said Pat. No. 3,366,337, mixed reactants within the chamber must be quickly removed before completion of the reaction and formation of a blockage in the gun. Means are therefore provided for introducing a stream of solvent into the rearward end of the chamber after the gun has been operated, so that the chamber and outlet orifice can be cleaned of residual material to enable further spraying operations. Disadvantages of the technique are that a separate container is usually required to collect the discharge during cleaning, it can happen that solvent contacts and mars a finished product, the use of solvent adds cost to the operation and it is undesirable for solvent to be sprayed into the air. In addition, cleaning the chamber with solvent often is less than thorough, with the result that the gun must be disassembled to remove hardened material, and whenever the gun is not going to be used for a period of time, customary practice contemplates that it be soaked in solvent to remove mixed reactants. Also, with some types of modern day materials, mere impingement together of reactants in a chamber does not provide satisfactory mixing.
One use for plural component spray guns is in fiberglass spraying systems in which resin and catalyst are brought together in a spray into which fiberglass particles are introduced for being wetted and carried to a workpiece. Fiberglass spray guns, often referred to as FRP (fiberglass reinforced plastics) spray guns, may be of either the internal or external mix type. As above discussed, internal mixing of coreactive liquid components such as catalyst and resin requires time consuming and extensive cleaning of the spray gun periodically and after each use. External mixing of the components, on the other hand, decreases the amount of cleaning required. With an external mix spray gun, catalyst and resin streams are emitted from separate nozzles and usually are atomized prior to being mixed together. Because the catalyst and resin are mixed externally of the spray gun, there are no mixed reactants within the gun which must be removed prior to completion of the reaction to prevent a blockage in the gun.
Some major concerns in designing and operating external mix FRP spray guns include providing a thoroughly mixed spray, a sufficiently high utilization of catalyst and low emissions of catalyst fumes. Unless the resin and catalyst are thoroughly mixed the fiberglass curing rate will not be uniform and there will be a lack of uniformity between workpieces. However, it is difficult to properly mix atomized sprays of resin and catalyst. Also, because the catalyst is atomized prior to mixing, a considerable amount of the catalyst spray does not become admixed with the resin spray and there is a decreased utilization of catalyst and an increase in catalyst fumes.
Typical prior spray guns having the aforementioned disadvantages are shown in U.S. Pat. Nos. Re 31,163 to Gardner and 4,618,098 to Hedger et al. and in British U.S. Pat. No. 735,983 to Dehn. In each, resin and catalyst are both atomized prior to being mixed, in consequence of which there is less than thorough mixing of the resin and catalyst, low catalyst utilization and relatively high emissions of catalyst fumes.